1. Field of the Invention
The disclosure relates to an air-fuel ratio control apparatus for an internal combustion engine and a method for controlling an air-fuel ratio.
2. Discussion of the Background
Japanese Unexamined Patent Application Publication No. 2011-144754 discloses an air-fuel ratio control apparatus having a function of determining an imbalance failure based on the output signal of an air-fuel ratio sensor provided in the exhaust system of an engine. This apparatus executes air-fuel ratio oscillation control to oscillate the air-fuel ratio at a predetermined frequency while the engine is in operation, and determines an imbalance failure using a decision parameter obtained by dividing a 0.5th-order frequency component intensity included in the output signal of the air-fuel ratio sensor by the component intensity of the predetermined frequency. The 0.5th-order frequency component is the component of a half of a frequency corresponding to the rotational speed of the engine. When an imbalance failure occurs, the intensity of the 0.5th-order frequency component increases. The greater the degree of imbalance is, the greater the value of the decision parameter becomes. It is therefore possible to determine an imbalance failure by comparing the decision parameter with a predetermined threshold value.